Vacuum Web Coating

Blogmaster: Dr. Charles A. Bishop

It occurred to me that in my travels to various companies I have seen all kinds of web handling, some good, some bad, and it might be worth commenting on some of the problems that can occur as a result of roll handling.

The first comment is that when you receive a roll you have little control over the handling it has had up to that point other than perhaps specifying how the roll is to be shipped to you. This can include the decision of how the rolls are formed such as are they wound flush to the core ends or does the core extend beyond the end of the roll. If the rolls are wound flush to the ends there are two methods of moving the roll around, one is to use a sling that the roll sits on, usually from an overhead crane, to support the roll. The other is to use a prong that slides into the core and supports the roll from the inside of the core. This may use an adapted fork lift truck to move the rolls around. Both can have their advantages and disadvantages. One of the most common problems is when using prongs and forklift trucks and that is that in locating the prong into the core the first attempt or two fail and the prong hits the end of the roll or end of the core and produce some damage. With slings it is common to use canvas or similar flexible material to support the outside of the roll. If this flexible material is not kept clean it can result in any debris in the sling being pressed into the surface of the roll. Depending on the size of the debris the damage may only affect a few layers but if the debris is large the damage may be seen through many hundred layers of thin polymer web. Moving rolls around by fork lift trucks where the standard two forks are used either side of the roll may also result in some damage where the roll sits on the edges of the forks. This too can vary in depth and may also be affected by the smoothness of the floor as if the forklift truck has to work on an uneven floor it will bounce up and down over the floor and when the roll bounces down onto the forks the damage can be greater than if the floor were even. If the roll is wound onto a core that is longer than the web width it is common for the ends of the core to be used to lift the roll. Again this can be an opportunity for the operators to damage the ends of the roll as they try to locate the lifting hooks. Part of the problem is that as roll gets bigger and the weights increase the lifting tackle increases in size and weight and even a small bump by the lifting gear can leave a significant dent in the roll.

The next decision is where to put the rolls once you have received them. I have seen rolls simply resting on the floor, including rolls tipped over and resting on their side. Either of these can lead to problems. Resting the roll directly onto the floor can again lead to any dirt on the floor being pressed into the roll. If the roll is placed down on its edge the dirt can be pressed into the gaps between the layers of the roll and then be carried into the vacuum system. Resting the roll on the floor, or any hard surface the roll may relax and when next lifted may have a flat on the roll where the roll contacted the floor. This flat will then result in tension fluctuations until the flat has disappeared. The size of the flat will depend on how hard or soft the roll has been wound as well as if there are any temperature or humidity changes during the time the roll is stored on the floor.

Choosing the time when to move a roll can be important. If the roll is moved from the delivery truck into storage area it can often be that the temperature and humidity will be different. Once the rolls have been moved into storage they will begin to change to equalise with the surrounding atmosphere. As different parts of the roll will change at different rates it is possible that the rolls can become unstable for a time during the transition and will be prone to telescoping if moved during this time. The duration of the time when the rolls are unstable will depend on the magnitude of the change that needs to take place. This situation also applies following the vacuum deposition process. It is also something that needs to be considered when deciding what type of packaging is to be used to ship the completed rolls.

A very practical point raised. In my opinion, Rolls with core extending beyond the ends of roll are easier to handle. It is therefore wise to put this condition at the time of ordering. About storing of rolls, you have rightly pointed out that if enough attention is not paid, the roll can get damaged. In India mostly rolls are stored in wooden pallets built specifically for this purpose. This avoids most problems mentioned by you. These pallets can be stored vertically to save space.

Blogmaster

Dr. Charles A. Bishop

Charles is a toolmaker by trade after completing a mechanical engineering apprenticeship. He then entered University and obtained a Bachelors degree in materials engineering with a Diploma in Industrial Studies. During his final year he first started work on vacuum based research, helping develop a process for manufacturing titanium based bone implants for tendon location. He went on to obtain a Masters degree and Doctorate following further research into vacuum deposition processes. During this time and as a postgraduate he also worked as a consultant.

Charles next spent time in industry working for various divisions of ICI including polyesters, nylon, Imagedata, Flex Products Inc., and explosives as well as contributing to other projects. In 1998 he took the opportunity to return to consultancy work and set up his own company.

Charles has more than 30 years experience in vacuum deposition mainly onto flexible webs. He has regularly contributed papers to conferences and recently has edited this blog on behalf of AIMCAL as well as being one of their presenters for various webinars and the more formal Converting School courses.

Charles has also published 2 books, Vacuum deposition onto webs, films and foils and Roll-to-roll vacuum deposition of barrier coatings.